“She also had First Minister’s Questions (FMQs) on Thursday and would not have been able to get back to Holyrood from Cambridge in time.

“She has taken part in all the other debates.”

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Angus Robertson (r) represented the SNP in the debate

The debate finished at 9pm on Wednesday and FMQs starts at 12pm every Thursday.

Also participating in last night’s debate was Plaid Cymru leader Leanne Wood, who is in the same position as Ms Sturgeon.

Ms Wood is a Welsh Assembly Member but is not an MP so is also not running for parliament in next Thursday’s General Election.

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Ms Sturgeon had to be back in Edinburgh for FMQs today, her spokesman said

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Amber Rudd was questioned by every panellist apart from Corbyn over where Mrs May was

While Ms Sturgeon has not been hauled over the coals publicly for not participating, several viewers noticed she was missing from the line-up.

Author Linda Grant, whose novel The Dark Circle won the Orange Prize, tweeted: “Any theories on why Nicola Sturgeon gets a pass for not turning up to the debate?”

Marilyn Anderton, added: “Yes it was a Leaders debate but not one of the cowardly leaders in the debate questioned where Sturgeon was, they just bullied Amber Rudd.”

Vanessa Christie, said: “Nicola Sturgeon was also absent from the debate and sent her deputy, no one seems to have mentioned that.”

Several people also reminded the BBC Mrs May was not the only leader who did not show up, after reports during BBC Breakfast and Radio 4's Today programme said it was a “leaders debate with one missing”.

Andrew Cuff, said: “@BBCBreakfast Nicola Sturgeon wasn’t at the debate either last night.”

Phillip Taylor, added: “@BBCr4today Why not saying Nicola Sturgeon also didn’t turn up for the debate?”